System, probe and method for measurement of fastener loading

ABSTRACT

In the present disclosure, embodiments including a system, probe and method are disclosed for accurately measuring the strain or extension of a fastener that occurs as the nut on the fastener is tightened and the fastener is put under load. The measurement technique is based on measurement of the time for an ultrasonic wave generated on one end of the fastener to travel a round trip through the fastener. As the fastener is tightened, the applied stress causes an associated increase in length. This length can be determined from a measurement of the increase in transit time. In various embodiments, the disclosed device and method uses laser ultrasonic testing (LUT), in which a pulsed laser generates the ultrasonic wave and a type of laser vibrometer detects the wave when it returns to the position of generation following a combination of longitudinal and shear wave reflections.

RELATED APPLICATIONS

This application claims the benefit of and is a continuation ofnon-provisional U.S. patent application Ser. No. 15/594,042, filed onMay 12, 2017. This application for patent claims the benefit ofprovisional application 62/350,251, filed on Jun. 15, 2016. Bothapplications are incorporated herein in their entirety.

FIELD OF THE INVENTION

This field of this disclosure relates generally to measuring fastenerload.

BACKGROUND

Measuring the load on a fastener during installation of the fastener isa long standing problem. As a general problem, the fastener load valueshould be optimized depending on the fastener, assembly materials andapplication. During a typical application, when a fastener is put intouse, it is inserted into clearance holes in a multi-part assembly. Awasher and nut are lightly threaded onto the end by hand. Tightening thefastener requires two people: one person fixing the head with a standardwrench and another person on the opposite side of the structuretightening the nut with a torque wrench. The nut is tightened to aspecified level of torque. In this example, the torque wrench measurestorque as a proxy for bolt load. This technique suffers from inaccuracydue to inconsistent or uncalibrated friction between the fastener andits mating hole. The torque applied must overcome the contact frictionas well as loading the fastener. In alternative fastener installations,one end of the fastener or bolt may be integrated into the assemblydirectly, however, a torque wrench measurement still suffers the sameshortcomings.

For assembly in critical areas, instrumented fasteners may be used.Instrumented fasteners have a transducer bonded to the head of thefastener. In typical applications, this transducer allows an ultrasonicmeasurement of the fastener extension, using a commercially availableinstrument for ultrasonic fastener load measurement. In availablecommercial ultrasonic instruments, ultrasonic vibration is inducedmechanically at the transducer end of the fastener. In a typical use,the instrument then measures the arrival time of the induced wave thattravels the length of the fastener, reflects from the opposite face andreturns to the starting point. As the fastener is tightened to applytensional stress, the fastener is put under positive strain andincreases in length. At the same time, the sound velocity decreasesthrough the acousto-elastic effect. An ultrasonic pulse propagatingthrough a loaded fastener thus propagates a greater distance at a slowervelocity than in an unloaded fastener, producing a time delay that canbe used to determine the internal stress in the shank of the fastener.

In a maintenance or service environment it is desirable to measure thecurrent load. However, there is no measurement reference point at zeroload, so the transducer approach cannot be used. The use of appliedtorque for load measurement suffers from the same limitations describedearlier.

Current commercial devices which perform transducer-based measurement offastener extension have several problems including difficult, complexand cumbersome use due in part to the transducer-contact interface,cost, a limitation of the accuracy of the device and that themeasurements were available in real time.

SUMMARY

In the present disclosure, embodiments including a system, device andmethods are disclosed for accurately measuring the load on a fastenerthat occurs as the nut on the fastener is tightened. The measurementtechnique is based on measurement of the time for an ultrasonic wavegenerated on one end of the fastener to travel a round trip through thefastener. As the fastener is tightened to apply tensional stress, thefastener is put under positive strain and increases in length. At thesame time, the sound velocity decreases through the acousto-elasticeffect. An ultrasonic pulse propagating through a loaded fastener thuspropagates a greater distance at a slower velocity than in an unloadedfastener, producing a time delay that can be used to determine theinternal stress or load in the shank of the fastener. In order todetermine the load from the change in arrival times, a model must firstbe developed that takes into account a number of factors including thelength and diameter of the fastener, the placement of the threads, thelongitudinal and shear velocity, the change in these velocities as afunction of load, the fastener temperature and the velocities at thistemperature and the internal path of the waves. As the load varieslinearly with the change in arrival time, all the other factors can beused to calculate a slope, so the load can be determined by multiplyingthe slope by the change in arrival time.

In alternative embodiments where the fastener load needs to be measuredin place without the benefit of measured reference signal, the user maybe able to look up a previously recorded reference signal for thespecific fastener. If this data is unavailable, then a combination of atleast three measured signals may be used to determine load.

In various embodiments, the disclosed device and method uses laserultrasonic testing (LUT), in which a pulsed laser generates theultrasonic wave and a type of laser vibrometer detects the wave when itreturns to the position of generation. Disclosed unexpected experimentalresults of ultrasonic reflected signal detection are incorporated intothe probe detection of fastener load for increased measurement accuracyand reliability.

In various embodiments, disclosed is a compact, fiber optic probe thatdelivers both laser beams (generation and detection) to the end surfaceof the fastener. In an embodiment disclosed, the probe has an adapter atits end that allows it to be threaded directly to the end of thefastener. Various embodiments include adapters designed for fasteners ofdifferent diameter and thread pitch. In various embodiments, the smallsize of the probe allows rapid mounting and dismounting and provides foreye safety by completely encapsulating the end face of the fastener.

In various embodiments, the probe optical components overlap the twolaser beams and focus them on the end surface of the fastener. Theoptical design of the probe is precise enough that no internal alignmentis required. In order to maintain focus, the probe is seated into adesigned standoff position after threading onto the fastener.

In various embodiments, the laser ultrasonic bandwidth available forprobe generated and detected signals range to at least 125 MHz,providing a very accurate measurement of transit time and thus fastenerextension. In various embodiments, the disclosed probe signal bandwidthprovides a substantial increase in accuracy over current transducerbased fastener load measuring devices due to the higher availablebandwidth.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a flow chart which outlines an embodiment methodology forlaser ultrasonic measurement of fastener loading.

FIG. 2 shows an exemplar embodiment of the laser probe and associatedmeasurement system components.

FIG. 3A shows a side view of an exemplar probe embodiment with afastener attached.

FIG. 3B shows a perspective view of an exemplar probe embodiment with afastener attached.

FIG. 3C shows a side cross section view of an exemplar probe embodimentwith a fastener attached.

FIG. 4 shows a side cross section view of a fastener superimposed withexemplar trajectories of laser ablation generated ultrasonic signaltrajectories.

FIG. 5 shows the output from the laser detector signal showing the peakreflected signal detected.

FIG. 6 shows the output from the laser detector signal showing variouspeaks of the reflected signal detected and an exemplar arrival signalselection window.

FIG. 7 shows an optics diagram for an exemplar embodiment of the laserprobe.

DETAILED DESCRIPTION

In various embodiments the round-trip travel times for a laser generatedultrasonic signal propagating the length of a fastener during unloadedand loaded conditions are determined by a cross-correlation of theidentified signal peaks in the acquired signals. In contrast toconventional ultrasonic transducer probes, in various embodiments, onlyoptical beams contact the end of the fastener, and the vibration isinduced in a small spot rather than the larger area occupied by atransducer. As explained below, these differences in ultrasonic signalgeneration produce significantly different temporal signalcharacteristics. In various embodiments, the signal peak utilized todetermine signal arrival time differences is not the expected directround trip signal, but one which has reflected multiple times off thefastener walls and converted on its last reflection from a longitudinalwave to a shear wave. Note that omnidirectional beams are cause both bythe small area of generation and by laser ablation effects.

In conventional fastener ultrasonic measurement systems, when atransducer is used to measure bolt extension, a 0° longitudinal wavetransducer is selected. This conventional mechanical transducergenerates a longitudinal wave that travels along the normal to the endsurface, i.e. along the axis of the fastener. This wave reflects fromthe opposite surface (at the opposite end of the fastener) and returnsto the point of generation, where it is detected. In this conventionalultrasonic transducer measurement, the path of the wave is simple andintuitive: it follows the central axis of the fastener.

In contrast to transducer based systems, in various embodiments of thepresent system, when generating ultrasonic waves in metals with a pulsedlaser, there are two generation regimes: thermoelastic at low pulseenergy and ablative at higher laser energy. The ablative regime ischosen for this application, as the longitudinal waves have much higheramplitude. In addition it is known that laser-generated longitudinalwaves in the ablative regime are nearly omnidirectional: they arestrongest along the normal to the surface, but the wave amplitude dropsonly gradually at larger angles to the surface normal. The broad angularrange of the generated waves leads to an unexpected result: thestrongest return signals correspond to waves that reflect several timeson the sides of the fastener, while the direct axial signal isrelatively weak. It is known that the strong signals correspond toreflected waves, as their arrival times are longer than the arrival timefor the direct axial wave. A longer arrival time means a longer pathlength. In various embodiments, by measuring the arrival times of thelargest signals, determining the path followed by the waves is possible.The strongest signals correspond to longitudinal waves that aregenerated off axis and reflect several times from the side walls as theypropagate to the opposite face of the bolt and back again. It has alsobeen observed that the very last reflection before returning to the endface converts the longitudinal wave to a shear wave. Such “modeconversion” happens at every point of reflection, but only theconversion at the last reflection is pertinent.

By way of explanation for these observed signal characteristics whichare utilized in the implementation of various embodiments, one mayconsider several physical reasons why off-axis waves are stronger andwhy mode conversion to a shear wave on the last reflection is favored:

If the waves are looked upon as rays, then only one ray travels straightdown the axis and back, while many rays (in the shape of a hollow cone)travel at a given angle to the axis. The cylindrical shape of thefastener captures all these off axis rays and bundles them together eachtime they pass through the axis of the fastener. Thus, off axis raysgive a stronger signal in a cylinder than the single ray traveling downthe axis.

The detection process at the end face uses a type of interferometer andis sensitive to out-of-plane motion of the surface. Thus, we mustconsider how the particle motion of the arriving waves couples toout-of-plane motion of the end surface. Longitudinal waves have theirparticle motion in the direction of propagation, while shear (ortransverse) waves have their particle motion perpendicular to thedirection of propagation.

As mentioned above the longitudinal waves traveling off-axis give largersignals when they rejoin on axis than the on-axis wave. However, as theangle to the axis increases, longitudinal waves couple poorly intoout-of-plane motion of the end surface. By contrast, off-axis shearwaves will couple better into out-of-plane motion of the end surface.This explains why the shear wave which converts at the last bounce ispreferred for utilization as a reference signal in various embodiments.

Shown in FIG. 1 is a flow chart outlining an exemplar methodology forvarious embodiments for measuring fastener head loading 101. Variousaspects and components of the method and apparatus are detailed furtherbelow and in the other drawings. The process begins by attaching theprobe shown in FIGS. 3A-3C 301 to a fastener used in an assembly. Asexplained below, in various embodiments, the probe is designed to beattached to the fastener 102 after installation of a nut or loadingcomponent. Prior to loading the fastener, a reference signal isgenerated and recorded 103. In various embodiments, a temporal window isselected either automatically or manually from the acquired referencesignal 104. In various embodiments, following the set-up phase 105, thereference signal detection is performed 122. In this phase, the highestpeak in the arrival window is selected either manually or automatically106 and the identified signal peak is smoothed by interpolation 107.During the loading phase 121, as the fastener is loaded 108, the probegenerates and detects (or acquires) an ultrasonic signal 109. As withthe reference signal, during the loading detection phase 120 the largestsignal peak is selected manually or automatically 111. In the lastphase, the strain and load calculation 119, the selected andinterpolated reference and loaded signals are cross-correlated 114 toobtain the time difference [delta T]. Then the methodology mayoptionally either use a finite element based model 116 to compute theload or a calibrated look-up table based on empirical data 117 tocompute the measured load, completing one cycle of measurement 118. Thecalibrated look-up table or model-based methodology incorporatescorrection of (1) the axial variation of the internal strain and stressof the fastener, (2) change in (shear and longitudinal) wave velocitywith stress or load, (3) variation of wave velocity with temperature and(5) the non-axial propagation of waves in the fastener as exhibited invarious embodiments disclosed herein.

FIG. 2 is a component diagram for various embodiment of the measurementsystem utilizing the laser ultrasonic probe 201. The system componentsin various embodiments are generally divided between the probe 201, thelaser ultrasonic generation components 202, the laser ultrasonicdetection components 203, and the acquired signal digitizer and computercontrol component 205. In various embodiments, a control system 240 isutilized to operate the ultrasonic signal generation by controlling apulsed Nd:YAG laser 208, which propagates the ablating laser pulsesthrough the generation optical fiber 209 to the probe 201.

In various embodiments, the returning ultrasonic waves are detectedusing a detection laser 230, which sends the detection laser beam 204 toa variable fiber splitter 220 which splits the laser input between areference beam 212 and detection beam 205. A receiver component 210receives the signal back 206 from the probe 201 and converts the signalfor output 211 for the digitizer.

FIGS. 3A-3C show various views of an embodiment of the laser ultrasonicfastener probe 301. In FIG. 3A, the probe is shown in its case 304 froma side view. The probe case 304 is affixed to the fastener 302 with theprobe collar 303 which is threaded onto the end of the fastener. Alsoshown are the connectors for the laser generation fiber 305 and laserdetection fiber 306. FIG. 3B shows a perspective view of the probe 301with a fastener 302 affixed to the probe by the probe collar 303. Theprobe collar is designed to thread onto the fastener without rotatingthe fastener or the probe. The diameter of the probe collar is designedto be large enough that internally-reflected shock waves from theablation process arrive sufficiently late in time to avoid disturbingthe desired signals.

FIG. 3C shows a side cross-sectional view of the probe 301 with anaffixed fastener 302. The probe collar 303 is shown seated against thefastener end, demonstrating how the design provides a rapid and simplecapability to mount the probe onto an assembly with the fastenerpositioned at a consistent distance from the probe optics. Also shownare the connectors for the laser generation fiber beam 305 and laserdetection fiber beam 306, and optical components the turning prism 309and dichromic beam combiner 307. Probe optics are further detailedbelow.

FIG. 4 shows exemplar ultrasonic wave trajectories within a fastener.Ultrasonic waves emanate and return to the focal point for the laserultrasonic generation and detection beams 401. Conventional transducergenerated ultrasonic waves (and one trajectory of waves produced by thelaser ultrasonic probe) have a predicable trajectory 402 which is twicethe length of the fastener, here identified as “2L” which stands for 2longitudinal wave segments. An exemplar trajectory consisting of 4longitudinal wave segments is shown 403, as is a trajectory showing modeconversion on the last reflected wave segment “2L1S” for 2 longitudinalsegments and 1 shear wave segment. Shown as an exploded view of the wavemode conversion point 405 in 405A are a depiction of the longitudinalwave 407 and shear wave 406 following the mode conversion.

FIG. 5 shows a graph 501 of an exemplar temporal signal 502 for anarrival reference or loaded signal with the peak detected reflected wavesignal peak identified 503. Shown is an exemplar arrival window 506which in various embodiments is determined either manually orautomatically by the selection of boundaries 504 and 505.

FIG. 6 shows a more detailed graph 601 of an exemplar temporal signaldetected by the laser ultrasonic probe. Shown is an exemplar arrivalwindow 612 which in various embodiments is determined either manually orautomatically by the selection of boundaries 610 and 611. For thisexemplar signal, the actual signal trajectories were calculated forvarious signal peaks including the conventional 2L detection 602, adetection of the 4L wave 603, a 6L1S wave 604, a 8L1S 605 wave, an 11L1Swave 607, a 12L1S wave 608, and the peak signal identified as the 10L1Swave 606. As can be seen from the exemplar detected signals in FIG. 6,the 2L signal 602 is difficult if not impossible to distinguish fromsignal noise. Solutions which rely upon the reflection of the simple 2Lsignal for determination of the length change thus suffer from theproblem of a correctly identified and an effective signal detection.

Probe optics for various embodiments, explained in more detail below areidentified in FIG. 7. The optical probe has two functions:

Optical: (1) each beam is delivered to the probe by its own fiber 702and 703, (2) deliver the generation and detection beams in a coaxial,overlapped configuration to the center of the end surface 710 and (3)focus both beams at the designed standoff position and with the designspot diameter values.

Mechanical: incorporate adapters at the output end of the probe thataccommodate fasteners with specific diameter and threaded pitch values,with all adapters positioning the end face at the desired standoffposition.

As shown in FIG. 7, the probe 701 incorporates several optical elements:

A lens to collimate each input beam including the generating collimatinglens 705 and the detection collimating lens 704;

A filter to block light from the generation laser from traveling intothe detection path also called the blocking filter 707;

A turning prism 708 to turn the detection beam by 90 degrees;

A dichroic element to combine the detection and generation beams 706;

An objective lens 709 that focuses the two beams at a specified standoffdistance and to specified spot diameters 711;

An easily replaceable output window 710 that protects the internaloptics from dust.

In various embodiments, the laser ultrasonic fastener load measurementprobe and system are utilized for fasteners in applications with one orboth ends of the fastener exposed for attachment of the probe. Inalternative embodiments the probe utilizes a connection interface to thefastener which may be affixed by threading the probe to the fastener. Inalternative embodiments the probe utilizes a connection interface to thefastener which may be affixed by snapping the probe onto the fastener.In alternative embodiments the probe utilizes a connection interface tothe fastener which may be affixed by mounting the probe onto tospecially designed wrench used for loading the fastener.

In various embodiments, the laser ultrasonic fastener load measurementprobe and system are utilized for fasteners in aerospace vehicleassembly, automotive assembly, critical building structural assembly,among other applications.

What has been described herein is considered merely illustrative of theprinciples of this invention. Accordingly, it is well within the purviewof one skilled in the art to provide other and different embodimentswithin the spirit and scope of the invention.

What is claimed is:
 1. A method for measuring a fastener loadcomprising: generating and directing a first laser ultrasonic generationbeam to a first end of the fastener when the fastener is unloaded;generating and directing a first laser ultrasonic detection beam to thefirst end of the fastener when the fastener is unloaded; acquiring by anoptical sensor a reference ultrasonic signal while the fastener isunloaded; loading the fastener; generating and directing a second laserultrasonic generation beam to the first end of the fastener when thefastener is loaded; generating and directing a second laser ultrasonicdetection beam to the first end of the fastener when the fastener isloaded; acquiring by the optical sensor a loaded ultrasonic signal whilethe fastener is loaded; determining a time difference between a peakidentified in the unloaded reference signal and a corresponding peakidentified in the loaded ultrasonic signal, wherein the peak identifiedin the unloaded reference signal and a corresponding peak identified inthe loaded ultrasonic signal are the result of a mixture of longitudinaland shear waves which reflect several times from at least one side walland once from an opposite end of the fastener prior to returning to adetection location on the first end of the fastener; calculating ameasured fastener load based at least in part on the time difference. 2.A method as in claim 1 wherein the generation and detection beams aredirected by a probe attached to the fastener.
 3. A method as in claim 1wherein the probe may be attached to a threaded end of the fastener. 4.A method as in claim 1 wherein the probe may be threaded onto a threadedend of the fastener.
 5. A method as in claim 1 wherein the probe may beattached to a rotationally torqued end of the fastener.
 6. A method asin claim 1 wherein the generation and detection beams are combined withan optical element.
 7. A method as in claim 1 wherein the generation anddetection beams impinge on overlapping areas of the fastener.
 8. Asystem for measuring a fastener load comprising: a generating lasercomponent which generates ultrasonic waves at a first end of thefastener; a detecting laser component which generates an optic beam usedfor detecting ultrasonic waves at the first end of the fastener; anoptical sensor; a probe which directs laser beams from the generatinglaser component and detecting laser component onto the first end offastener; wherein the generating laser component generates by a laser, areference ultrasonic signal in the fastener when the fastener isunloaded and generates a loaded ultrasonic signal in the fastener whenthe fastener is loaded; wherein the detection laser component andoptical sensor acquires the detected reference ultrasonic signal at thefirst end of the fastener when the fastener is unloaded and acquires theloaded ultrasonic signal at the first end of fastener when the fasteneris loaded; wherein a peak in the reference ultrasonic signal isidentified and a corresponding peak in the loaded ultrasonic signal isidentified, wherein the peak identified in the unloaded reference signaland a corresponding peak identified in the loaded ultrasonic signal arethe result of a mixture of longitudinal and shear waves which reflectseveral times from at least one side wall and once from an opposite endof the fastener prior to returning to a detection location on the firstend of the fastener; wherein the fastener load is determined at least inpart on the time difference between the identified reference signal peakand the identified loaded signal peak.
 9. A system as in claim 8 whereinthe probe may be attached to a threaded end of the fastener.
 10. Asystem as in claim 8 wherein the probe may be threaded onto a threadedend of the fastener.
 11. A system as in claim 8 wherein the probe may beattached to a rotationally torqued end of the fastener.
 12. A system asin claim 8 wherein the generation and detection beams are combined withan optical element.
 13. A system as in claim 8 wherein the generationand detection beams impinge on overlapping areas of the fastener.
 14. Aprobe for measuring a fastener load comprising: a first input receptaclefor a generating laser component which generates ultrasonic waves in thefastener; a second input receptacle for a detecting laser componentwhich generates optic beams used for detecting ultrasonic waves in thefastener; optics which direct laser beams from the generating lasercomponent and detecting laser component overlapped onto a first end ofthe fastener; wherein the generating laser component generates by alaser, a reference ultrasonic signal in the fastener when the fasteneris unloaded and generates a loaded ultrasonic signal in the fastenerwhen the fastener is loaded; wherein the detection laser component andoptical sensor acquires the detected reference ultrasonic signal in thefastener at the first end when the fastener is unloaded and acquires theloaded ultrasonic signal in the fastener at the first end when thefastener is loaded; wherein a peak in the reference ultrasonic signal isidentified and a corresponding peak in the loaded ultrasonic signal isidentified, wherein the peak identified in the unloaded reference signaland a corresponding peak identified in the loaded ultrasonic signal arethe result of a mixture of longitudinal and shear waves which reflectseveral times from at least one side wall and once from an opposite endof the fastener prior to returning to a detection location on the firstend of the fastener; wherein the fastener load is determined at least inpart on the time difference between the identified reference signal peakand the identified loaded signal peak.
 15. A probe as in claim 14wherein the probe may be attached to a rotationally torqued end of thefastener.
 16. A probe as in claim 14 wherein the generation anddetection beams are combined with an optical element.
 17. A probe as inclaim 18 wherein the optical element is a dichroic mirror.
 18. A probeas in claim 14 wherein the generation and detection beams impinge onoverlapping areas of the fastener.
 19. A non-transitory computerreadable medium storing a program for measuring a fastener load,comprising instructions, which when executed on a computer cause thecomputer to carry out the steps of: determining a time differencebetween a peak identified in the unloaded reference signal and acorresponding peak identified in the loaded ultrasonic signal, whereinthe peak identified in the unloaded reference signal and a correspondingpeak identified in the loaded ultrasonic signal are the result of amixture of longitudinal and shear waves which reflect several times fromat least one side wall of the fastener and once from an opposite end ofthe fastener prior to returning to a detection location on a first endof the fastener; calculating a measured fastener load based at least inpart on the time difference.